This invention relates to a method and apparatus for evaluating surface features of a substrate such as wafers, reticles, photomasks, and the like (hereafter referred to generally as substrates). More particularly, the present invention relates to an optical inspection system that can produce images of such substrates with minimum color variation.
Integrated circuits are made by photolithographic processes, which use photomasks or reticles and an associated light source to project a circuit image onto a silicon wafer. The presence of defects on the surfaces of the wafers is highly undesirable and adversely affects the resulting circuits. The defects can be due to, but not limited to, a portion of the pattern being absent from an area where it is intended to be present, a portion of the pattern being present in an area where it is not intended to be, chemical stains or residues from the wafer manufacturing processes, particulate contaminates such as dust, resist flakes, skin flakes, erosion of the photolithographic pattern due to electrostatic discharge, artifacts in the wafer such as pits, scratches, and striations in the substrate or pattern layer. Since it is inevitable that defects will occur, these defects have to be found and repaired prior to use. Blank substrates can also be inspected for defects prior to patterning.
Defects also occur in the device itself as the result of reticle defects and/or processing defects, for example. These defects may take the form of particles randomly localized on the surface, scratches, process variations such as under etching, etc. These and similar problems often arise when processing equipment malfunctions or degrades in performance over time. Examples of such equipment include plasma etchers, deposition systems, chemical mechanical planarization systems, reticle processing, and photolithography equipment. Obviously, a manufacturer needs to know when the process equipment ceases to function in an acceptable manner.
Many optical and electronic inspection systems and inspection techniques exist for identifying and classifying defects such as those on a partially or fully fabricated integrated circuit or a reticle. Such techniques and apparatus are well known in the art and are embodied in various commercial products such as many of those available from KLA-Tencor Corporation of San Jose, Calif. The simplest of these techniques involves a casual visual inspection by a technician of a wafer held in white light and examined to determine whether there is any variation in the appearance of the various dies fabricated on the wafer. Ideally, each die should have the same appearance when moved about under a white light. If there is any variation in the appearance of one or more of the dies on the wafer, then it can be assumed the dies are not structurally identical and some problem exists.
A related technique involves performing optical microscopy (e.g., bright field or dark field imaging) on the various dice of a wafer. Two images are formed from light reflected from two dice, and the images are compared by the microscopy tool. Any significant variation in the image of the dices indicates that there is a defect in at least one of the dice. Unfortunately, the intensity of light that is reflected from a wafer is affected by numerous inherent factors of the material being inspected. A typically inspected material is a dielectric (e.g., SiO2) thin film deposited over a metal or silicon substrate that has unintended thickness variation which occurs during fabrication. The reflectance of the wafer changes with the SiO2 thickness as a result of optical interference. This is seen in an image taken from that area of the wafer as gray level variation and is referred to as xe2x80x9ccolor variation.xe2x80x9d Since bright field inspection systems use images to find defects, such color variation is a nuisance source because it can cause false detection of defects. Color variation becomes more severe for SiO2 on Si or SiO2 on Cu interfaces, as compared with SiO2 on Al interface.
One conventional technique for reducing color variation is broadband illumination. The illumination beams are typically configured to have a relatively large wavelength range, i.e. bandwidth, in order to reduce coherence length. An inspection system will typically include a broadband illumination source capable of producing a 70 to 80 nm wide spectrum in the near ultraviolet (near UV) region or a about 150 nm wide spectrum in the visible wavelength region. Such illumination spectra results in a significant reduction in interference induced color variation for a film thickness greater than 1 xcexcm.
Although current broadband inspection systems work well for film thicknesses over 1 xcexcm, the wavelength range of the broadband illumination beam would have to be significantly extended (e.g., by a factor of two) to achieve sufficient reductions in color variation for film thickness less than 1 xcexcm. This bandwidth extension may not be practical on inspection machines which are designed to operate in their original wavelength ranges. These conventional broadband inspection systems result in color variation that is still excessively high (e.g., about 25% for 0.4 to 1.0 xcexcm thick SiO2 film on Si or copper which is becoming increasingly common) for thin films and results in many nuisance defects being detected as xe2x80x9crealxe2x80x9d defects. That is, the illumination source of an inspection machine may be limited to a smaller wavelength range than is required for achieving a sufficient reduction in color variation for thin films using conventional broadband illumination techniques.
Accordingly, there is a need for improved mechanisms for controlling color variation for thin films during optical inspection of a sample, such as a semiconductor wafer without extending the existing wavelength range of illumination sources available in inspection systems.
Accordingly, mechanisms are provided for controlling the spectrum of an illumination light beam of an optical inspection system so as to control color variation on an image of a film. The inspection system may thereafter use the controlled beam to generate an image of a film on a sample, where color variation in the image of the film is either suppressed or increased for a selected thickness or range of thickness value(s) of the film, as compared with using an uncontrolled illumination beam. By way of example implementations, the spectrum of the illumination beam is optimized for color variation reduction or pattern contrast (contrast between the circuit patterns and their neighboring areas) enhancement after the beam originates from a light source of the inspection system and before it reaches the sample or after at least a portion of the beam reflects off sample and before it hits an imaging device of the system. The controlled spectrum of the illumination beam has the same wavelength range as the uncontrolled spectrum. That is, the existing spectrum of the illumination beam used for the inspection system is controlled to minimize or maximize color variation of the image for particular thickness value(s) of the film without extending its wavelength range.
In one embodiment, a method of designing an optical spectrum of an illumination light beam within an optical inspection system is disclosed. A set of conditions for inspecting a film on a sample by directing an illumination light beam at the sample is determined. At least a portion of the illumination light beam is reflected off the sample and used to generate an image of at least a portion of the film on the sample. A plurality of peak wavelength values are determined for the optical spectrum of the illumination light beam so as to control color variation in the image of the film portion. The determination of the peak wavelengths is based on the determined set of conditions and a selected thickness range of the film. The determined peak wavelengths are wavelengths within the spectrum that have a maximum intensity value as compared to neighboring wavelengths. The determined peak wavelengths are also within a wavelength range of a light source of the illumination beam. In one specific embodiment, the color variation is reduced, while in another embodiment the color variation is increased to enhance pattern contrast.
In a specific implementation, the conditions include a wavelength range, a material type of the sample, an objective numerical aperture of the inspection system, and a detected spectral signal response of the inspection system. In a further implementation, a width and a height associated with each determined peak wavelength are determined. In one embodiment, the heights associated with the peak wavelengths are determined through apodization and a correction factor is applied to each height to compensate for wavelength dependence of color variation.
In another aspect, the invention pertains to a computer system operable to design an optical spectrum of an illumination light beam within an optical inspection system. The computer system includes one or more processors and one or more memory. At least one of the processors and memory are adapted to perform one or more of the above described methods. In yet another aspect, the invention pertains to a computer program product for designing an optical spectrum of an illumination light beam within an optical inspection system. The computer program product includes at least one computer readable medium and computer program instructions stored within the at least one computer readable product configured to cause a combining device to perform one or more of the above described methods.
In an alternative embodiment, an inspection system for analyzing a sample is disclosed. The system includes a light source for generating a illumination light beam, a first optics arrangement for directing the illumination beam to a film on a sample, and a second optics arrangement for receiving a portion of the illumination beam that reflected off the sample to thereby generate an image of the film. The inspection system further includes a spectrum controller for controlling a spectrum of the illumination beam so that color variation of the image is controlled (e.g., enhanced or suppressed). The controlled spectrum includes a plurality of peak wavelengths selected to control color variation for a particular thickness value of the film and a particular configuration of the inspection system. The peak wavelengths are between a wavelength range of a light source of the illumination beam. The determined peak wavelengths are also wavelengths within the spectrum that have a maximum intensity value as compared to neighboring wavelengths.
In one specific implementation, the particular inspection configuration includes a wavelength range, an objective numerical aperture of the inspection system, and a configuration of the first and second optics arrangements. In another embodiment, a plurality of heights associated with the peak wavelengths have a generally apodized distribution and compensate for a wavelength dependence of color variation. In yet another implementation, a plurality of widths associated with the peak wavelengths are selected to control color variation.
In a specific implementation, the spectrum controller is a filter in an optical path of the optical illumination beam, and the filter is positionable or positioned in an optical path of the illumination beam between the light source and the sample. In another embodiment, the filter is positionable or positioned in an optical path of the reflected illumination beam between the sample and an imaging device of the inspection system. In yet another embodiment, the filter is integrated within the first and/or second optics arrangement. In an alternative embodiment, the filter is an interference spectrum filter or a spatial light modulator combined with a wavelength dispersion device.
In one implementation of the inspection system, the light source is configured to generate a plurality of laser or narrow band beams, and the spectrum controller is configured to combine the laser or narrow band beams and adjust an intensity level of each of the plurality of beams so as to substantially produce the controlled spectrum of the illumination beam having the plurality of peak wavelengths with associated heights that have a generally apodized distribution and compensate for a wavelength dependence of color variation.
In another specific implementation of the inspection system, the light source is configured to generate a laser or narrow band beam, and the spectrum controller contains a Raman scattering material positioned or positionable to generate a plurality of peak wavelengths, and the spectrum controller is configured to adjust an intensity level associated with each of the plurality of peak wavelengths so as to substantially produce the controlled spectrum of the illumination beam having the plurality of peak wavelengths associated with heights that have a generally apodized distribution and compensate for a wavelength dependence of color variation.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.